The viscosity-temperature relationship of a lubricating oil is one of the critical criteria which must be considered when selecting a lubricant for a particular application. Viscosity Index (VI) is an empirical, unitless number which indicates the rate of change in the viscosity of an oil within a given temperature range. Fluids exhibiting a relatively large change in viscosity with temperature are said to have a low viscosity index. A low VI oil, for example, will thin out at elevated temperatures faster than a high VI oil. Usually, the high VI oil is more desirable because it has higher viscosity at higher temperature, which translates into better or thicker lubrication film and better protection of the contacting machine elements. In another aspect, as the oil operating temperature decreases, the viscosity of a high VI oil will not increase as much as the viscosity of a low VI oil. This is advantageous because the excessive high viscosity of the low VI oil will decrease the efficiency of the operating machine. Thus high VI oil has performance advantages in both high and low temperature operation. VI is determined according to ASTM method D 2270-93 [1998]. VI is related to kinematic viscosities measured at 40° C. and 100° C. using ASTM Method D 445.
PAOs comprise a class of hydrocarbons manufactured by the catalytic oligomerization (polymerization to low molecular weight products) of linear α-olefins (LAOS) typically ranging from 1-hexene to 1-octadecene, more typically from 1-octene to 1-dodecene, with 1-decene as the most common and often preferred material. Such fluids are described, for example, in U.S. Pat. No. 6,824,671 and patents referenced therein, although polymers of lower olefins such as ethylene and propylene may also be used, especially copolymers of ethylene with higher olefins, as described in U.S. Pat. Nos. 4,956,122 or 4,990,709 and the patents referred to therein.
High viscosity index polyalpha-olefin (HVI-PAO) prepared by, for instance, polymerization of alpha-olefins using reduced metal oxide catalysts (e.g., chromium) are described, for instance, in U.S. Pat. Nos. 4,827,064; 4,827,073; 4,990,771; 5,012,020; and 5,264,642. These HVI-PAOs are characterized by having a high viscosity index (VI) of about 130 and above, more preferably 150 and above, still more preferably 160 and above, yet still more preferably 200 and above, and one or more of the following characteristics: a branch ratio of less than 0.19, a weight average molecular weight of between 300 and 45,000, a number average molecular weight of between 300 and 18,000, a molecular weight distribution of between 1 and 5, and pour point below −15° C. Measured in carbon number, these molecules range from C30 to C1300. Viscosities of the HVI-PAO oligomers measured at 100° C. range from 3 centistokes (“cSt”) to 15,000 cSt. These HVI-PAOs have been used as basestocks in engine and industrial lubricant formulations. See also U.S. Pat. Nos. 4,180,575; 4,827,064; 4,827,073; 4,912,272; 4,990,771; 5,012,020; 5,264,642; 6,087,307; 6,180,575; WO 03/09136; WO 2003071369A; U.S. Patent Application No. 2005/0059563; WO 00/58423; and Lubrication Engineers, 55/8, 45 (1999); and have recently been found to be useful for industrial oil and grease formulations (e.g., U.S. patent application Ser. No. 11/172,161, filed Jun. 29, 2005.
Another advantageous property of these HVI-PAOs is that, while lower molecular weight unsaturated oligomers are typically and preferably hydrogenated to produce thermally and oxidatively stable materials, higher molecular weight unsaturated HVI-PAO oligomers useful as lubricant are sufficiently thermally and oxidatively stable to be utilized without hydrogenation and, optionally, may be so employed.
As used herein, the term “polyalpha-olefin” includes PAOs and HVI-PAOs. Depending on the context, the term “PAO” may include HVI-PAOs or it may be used to distinguish non-HVI-PAOs from HVI-PAOs. Generally, when PAO is used alone, it implies the products have properties similar to the fluids made from conventional polymerization process using BF3 or AlCl3 or their modified versions, as described in U.S. Pat. No. 6,824,671 and references therein.
Polyalpha-olefins of different viscosity grades are known to be useful in synthetic and semi-synthetic lubricants and grease formulations. See, for instance, Chapters 19 to 27 in Rudnick et al., “Synthetic Lubricants and High-Performance Functional Fluids”, 2nd Ed. Marcel Dekker, Inc., N.Y. (1999). Compared to the conventional mineral oil-based products, these PAO-based products have excellent viscometrics, high and low temperature performance. They usually provide energy efficiency and extended service life.
In the production of PAOs and HVI-PAOs, the feed is usually limited to one specific alpha-olefins, usually 1-decene. Occasionally, when 1-decene is not available in large enough quantity, small to moderate amounts of 1-octene or 1-dodecene is added to make up the quantity. It is generally thought that 1-decene is the most preferred feed (see reference “Wide-Temperature Range Synthetic Hydrocarbon Fluids” by J. A. Brennen, Ind. Eng. Chem. Prod. Res. Dev., 19, 2-6 (1980). When mixtures of feed are used, the products tend to be blocky copolymers rather than random copolymers and/or products produced at the beginning of the process are different than that produced at the end of the process, and the inhomogeneous polymer product will be characterized by poor viscosity indices (VI) and poor low temperature properties are produced. Thus, in the past, PAOs and HVI-PAOs have generally been made using pure C10 feeds.
There are specific examples of mixed feeds being used. For instance, in U.S. Pat. No. 6,646,174, a mixture of about 10 to 40 wt. % 1-decene and about 60 to 90 wt. % 1-dodecene and are co-oligomerized in the presence of an alcohol promoter. Preferably 1-decene is added portion-wise to the single oligomerization reactor containing 1-dodecene and a pressurized atmosphere of boron trifluoride. Product is taken overhead and the various cuts are hydrogenated to give PAO characterized by a kinematic viscosity of from about 4 to about 6 at 100° C., a Noack weight loss of from about 4% to about 9%, a viscosity index of from about 130 to about 145, and a pour point in the range of from about −60° C. to about −50° C. See also U.S. Pat. Nos. 4,950,822; 6,646,174; 6,824,671, 5,382,739 and U.S. Patent Application No. 2004/0033908. All these copolymers or co-oligomers produced by conventional Friedel-Crafts catalysts usually are characterized by having extra relatively short branches, such as methyl and ethyl short side chains, even though the feed olefins do not contain these short branches. This is because the Friedel-Crafts catalyst partially isomerizes the starting alpha-olefins and the intermediates formed during the oligomerization. The presence of short chain branches is less desirable for superior lubricant properties, including VI and volatility. In contrast, the copolymers described in this invention will not have extraneous short chain branches. If the feed is propylene and 1-dodecene, the predominant side chain in the polymers will be methyl and n-C10H23 side chains. Except for the contribution of usually less than 5% of the polymer end groups initiated through the rare allylic hydrogen abstraction of the alpha-olefin monomers by the active metal centers, the oligomers will not have extra ethyl, propyl, butyl, etc. side chains that are present in non-metallocene (e.g. Friedel-Crafts) methods.
Previous patents report the use of mixed alpha-olefins as feeds to produced co-oligomers or copolymers for use as lubricant components. U.S. Pat. No. 4,827,073 reported the use of a reduced chromium oxide on silica gel as catalyst to polymerize C6 to C20 alpha-olefins. Although liquid copolymers were produced by the process, the copolymer has very different polymer composition from the monomer ratio in the feed. The reduced chromium oxide on silica gel catalyst polymerized the lower alpha-olefins, such as 1-butene or 1-hexene, at a significantly higher rate than the alpha-olefins of 1-decene, 1-dodecene or larger alpha-olefins [see comparative examples in Example section]. As a result, the copolymer tends to be more blocky or more inhomogeneous in a conventional synthesis process. Both are detrimental to the product VI and low temperature properties. Similarly, Ziegler or Ziegler-Natta type catalysts have also been reported to copolymerize mixed alpha-olefins. Examples are U.S. Pat. Nos. 4,132,663, 5,188,724 and 4,163,712. The problem with using Ziegler or Ziegler-Natta catalysts is that they can only produce polymers of very high molecular weights. As a result, the products are used as plastics and additives, but are not suitable as high performance base stocks. Furthermore, according to all literature reports, Ziegler or Ziegler-Natta catalysts usually have higher reactivities toward smaller alpha-olefins, such as propylene, 1-butene, 1-pentene or 1-hexene, than toward larger alpha-olefins, such as 1-decene, 1-dodecene, or larger 1-olefins (reference Macromolecular Chemistry and Physics, 195, 2805 (1994) or 195, 3889 (1994)). This difference in catalyst reactivity resulted in heterogeneous chemical structures for the copolymers, which are not random copolymers and have high degree of blockiness. Both characteristics are detrimental for lube properties.
It would be highly beneficial if a process could be devised whereby a homogeneous and uniform PAO and/or HVI-PAO having an excellent viscosity-temperature relationship could be produced from a wide variety of mixed feed LAOS.
The present inventors have discovered an unanticipated method of producing a uniform PAO and/or HVI-PAO product by contacting a mixed feed of LAOs of varying carbon numbers with an activated metallocene catalyst.